Add code linting with luacheck

With luacheck mistakes in Lua code can be found, e.g. the use of undefined variables, and the code style can be checked.
For the github workflow I copied Panquesito7's check-release.yml file.

.github/workflows/check-release.yml

@@ -0,0 +1,15 @@
+on: [push, pull_request]
+name: Check & Release
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: apt
+      run: sudo apt-get install -y luarocks
+    - name: luacheck install
+      run: luarocks install --local luacheck
+    - name: luacheck run
+      run: $HOME/.luarocks/bin/luacheck ./